Leptin administration for the relative leptin deficiency in women with hypothalamic amenorrhea appears to improve reproductive, thyroid, and growth hormone axes and markers of bone formation, suggesting that leptin, a peripheral signal reflecting the adequacy of energy stores, is required for normal reproductive and neuroendocrine function.

Resistin is a novel adipocyte-secreted hormone proposed to link obesity with diabetes. Studies in mice have revealed conflicting data however, and the physiological role of circulating resistin in humans remains unknown. We conducted cross-sectional studies in 123 middle-aged women and 120 healthy young subjects and found that serum resistin levels did not correlate with markers of adiposity, including body mass index, waist-to-hip ratio, or fat mass, or insulin resistance assessed by homeostasis model, lipid profile, or serum leptin levels; but females had higher resistin levels than males (P < 0.02). We also found no difference in serum resistin levels between lean healthy and obese insulin-resistant nondiabetic and type 2 diabetic adolescents. Finally, to evaluate the effect of food deprivation and/or leptin administration on resistin levels, we performed interventional studies that revealed no significant difference in resistin levels after 48 h of fasting and/or leptin administration at either physiological or pharmacological doses. We conclude that circulating resistin is unlikely to play a major role in insulin resistance or energy homeostasis in humans.

Adiponectin is an adipocyte-secreted protein that circulates in high concentrations in the serum and acts to increase insulin sensitivity. Previous studies have shown that serum adiponectin is inversely associated with fat mass and insulin resistance in humans and that acute fasting decreases adipose tissue adiponectin mRNA expression in rodents. Whether acute energy deprivation, body fat distribution, or serum hormone levels are associated with circulating adiponectin in humans remains largely unknown. To identify predictors of serum adiponectin levels, we evaluated the association of adiponectin with several anthropometric, metabolic, and hormonal variables in a cross-sectional study of 121 women without a known history of diabetes. We also performed interventional studies to assess whether fasting for 48 h and/or leptin administration regulates serum adiponectin in healthy men and women. Our cross-sectional study shows that, in addition to overall obesity, central fat distribution is an independent negative predictor of serum adiponectin and suggests that adiponectin may represent a link between central obesity and insulin resistance. In addition, estradiol is negatively and independently associated with adiponectin, whereas there is no association between serum adiponectin and leptin, cortisol, or free testosterone levels. Our interventional studies demonstrate that neither fasting for 48 h, resulting in a low leptin state, nor leptin administration at physiological or pharmacological doses alters serum adiponectin levels. Further studies are needed to fully elucidate the physiology of adiponectin in humans and its role in the pathogenesis of insulin-resistant states.

To elucidate the role of leptin in regulating neuroendocrine and metabolic function during an acute fast, six to eight healthy, lean men were studied under four separate conditions: a baseline fed state and three 72-hour fasting studies with administration of either placebo, low-dose recombinant-methionyl human leptin (r-metHuLeptin), or replacement-dose r-metHuLeptin designed to maintain serum leptin at levels similar to those in the fed state. Replacement-dose r-metHuLeptin administered during fasting prevents the starvation-induced changes in the hypothalamic-pituitary-gonadal axis and, in part, the hypothalamic-pituitary-thyroid axis and IGF-1 binding capacity in serum. Thus, in normal men, the fall in leptin with fasting may be both necessary and sufficient for the physiologic adaptations of these axes, which require leptin levels above a certain threshold for activation. In contrast to findings in mice, fasting-induced changes in the hypothalamic-pituitary-adrenal, renin-aldosterone, and growth hormone–IGF-1 axes as well as fuel utilization may be independent of leptin in humans. The role of leptin in normalizing several starvation-induced neuroendocrine changes may have important implications for the pathophysiology and treatment of eating disorders and obesity